The present invention relates to half-eye spectacles.
Half-eye spectacles are spectacles where only that part of the spectacle lens is fitted into the spectacle frame which is not used for distance vision. With the known half-eye spectacles, spherical or toric single vision lenses are used where the "upper lens half" has simply been left off.
The optical power of the known half-eye spectacles is chosen in such a way that it will allow the spectacle wearer clear vision at short distances, i.e. at typical reading distances of about 40 cm. However, this means that the known half-eye spectacles have the disadvantage that they do not permit vision at intermediate distances, that is, for example, at distances of about one meter, with the accustomed visual comfort.
The object of the present invention is to provide half-eye spectacles which will also support visual accommodation in the intermediate area.
In accordance with this invention the focal power of the half-eye spectacle lens increases progressively from the upper rim area towards the bottom of the lens to that value which the spectacle wearer requires for near vision. At least one surface contributes to this increase in the focal power.
In an advantageous further development of the present invention, the prismatic power at the upper rim of the half-eye spectacles is zero so that no irritating image jump occurs when the gaze of the spectacle wearer is lowered (or raised) when he changes his visual direction from distance vision to near vision (or vice versa).
The power of the spectacle lens in the upper rim area may be 0 D. It is, however, particularly advantageous if the spectacle lens has a power other than zero in the upper rim area as the typical wearer of half-eye spectacles, an emmetropic presbyope, looks over the upper rim of the half-eye spectacles when looking into the distance. Only when his gaze is lowered, when he will typically want to see objects at medium visual distances, will the half-eye spectacle wearer look through the upper part of the progressive half-eye spectacles. Half-eye spectacles with a power other than zero in the area of their upper rim will then allow him clear vision for these finite distances, which the spectacle wearer can no longer manage alone through accommodation.
Furthermore, an upper rim area with power other than zero also has the advantage that it increases the "effective" addition of the spectacle lens--in contrast to the "surface theoretical" addition--by the value of the power in the upper rim area. This means that half-eye spectacles with an addition, i.e. with a difference between the power at the near reference point and the distance reference point (no longer present with half-eye spectacles) of 2 D and a power of 1 D in the upper rim area, will have the same power for the spectacle wearer as a surface with an addition of 3 D and a power of 0 D in the upper rim area. However, such a power of about 1 D is required by the emmetropic presbyope just for clear vision at infinite distances of about 1 m. In this way, he will find the corresponding powers on the surface of the half-eye spectacles for this distance and all short distances.
Nevertheless, the progressive surface of the half-eye spectacles still must only be calculated as if it had an addition of only 2 D. Such a surface with an addition of (for example) only 2 D has the advantage over a surface with an addition of (for example) 3 D that the progression corridor is wider: According to Minkwitz' proposition the width of the progression corridor (or the so-called progressive channel) is namely inversely proportional to the power increase.
Naturally both surfaces of the spectacle lens according to this invention for half-eye spectacles can contribute to the power increase from the upper rim area towards the lower portion of the lens. This is, however, of particular advantage if only one surface contributes to this power increase while the other surface has a spheric or toric design. In this way not only is an especially simple construction of the spectacle lens achieved, but it is, for example, also possible to have astigmatism correction.
In principle, any known progressive spectacle lens with the distance portion "cut off" can be used for the spectacle lens according to the present invention. One suitable, known progressive spectacle lens is, for example, described in the German DE-OS 28 14 916 (and explicit reference is herewith made to this publication with regard to all specifications, definitions, etc. not explained in detail here).
However, it is particularly advantageous if a progressive spectacle lens is used for the progressive spectacle lens in accordance with this invention as is described in the older German patent application (not published earlier) P 34 30 334. In this patent application, it is recognised that a progressive surface must inevitably show a certain amount of astigmatism. This astigmatism can either be concentrated in certain parts of the surface or be distributed more or less uniformly over the whole surface. In accordance with the present invention, it has now been recognized that a progressive spectacle lens particularly suitable for vision at close and intermediate distances can be obtained neither by distributing astigmatism uniformly over the surface, nor by transposing the astigmatism into the lateral lower rim areas and so into the progression zone; as in both cases the astigmatism, together with the increase in the surface focal power required in the progression corridor, will narrow the progression zone to only a narrow channel which permits clear vision. It has now been recognized that it is both required and possible for a progressive spectacle lens which permits clear vision primarily in the near and intermediate areas over a large part of the surface to transpose the main portion of surface astigmatism into a sector where the power practically does not increase. In this way, a progressive corridor is obtained which is largely free of astigmatism, comparatively wide and which permits clear vision in the intermediate area. If now the distance portion is selected as that area into which astigmatism is to be transposed, then a spectacle lens is obtained (as, of course, the distance portion is omitted for half-eye spectacles) which shows comparatively low astigmatism on all areas used and so offers optimum conditions for clear vision.
In a preferred version example of the spectacle lens for half-eye spectacles only one surface contributes to the power increase and this surface is designed in accordance with the older German patent application Ser. No. P34030 334; the distance portion, i.e. the area used for distance vision and so that area which is omitted with half-eye spectacles, has been selected as that area into which the main portion of astigmatism is transposed. For this reason, no compensation of the astigmatism which has been "pushed out" of the used area of the surface is necessary. As a result, it is also no longer necessary to give the second surface an aspheric design and so a purely spherical or toric surface can be used. By using a surface which contributes to the power increase in accordance with the German patent application Ser. No. P34 30 334, a progressive lens for half-eye spectacles is obtained which shows only low (unwanted) astigmatism in that area in which the power increases and which has practically no astigmatism in the reading portion.
Above all, however, the reading portion of this progressive surface is substantially larger than that of known progressive surfaces so that problem-free vision is possible over a large area "close-up" without any head movements. This progressive surface provided according to the present invention also has the following further advantage: As is known, emmetropic presbyopes show increased sensitivity to distortion. In addition, the typically much larger corneal vertex distance of half-eye spectacles has the effect of narrowing the lens areas which are free of aberration. With the conventional progressive lenses of the known designs this results in particularly the progressive channel being narrowed in such a way that it can hardly be used for clear vision. The progressive surface provided for in accordance with this invention with its new surface design is also a help here.
The surface of the spectacle lens in accordance with the present invention which contributes to the power increase (apart from the fact that it is designed in such a way so as to transpose the astigmatism into a certain area or certain areas) can have a similar construction to that of known lenses.
The calculation of the surface of the spectacle lens in accordance with the present invention for half-eye spectacles which contributes to the power increase can, as already stated in P 34 30 334.1, be done using periodic functions such as are given in DE-OS 28 14 916.
In addition, the surfaces which contribute individually to the power increase can also be put together piece by piece with a constant second derivative. Or surface properties can be pre-determined and the surfaces then calculated using Spline functions.
In this process, calculation procedures known per se can be used where first the progression corridor is optimised with no attention being paid to the surface properties of the distance portion.
The progressive spectacle lens in accordance with the present invention can also be designed without problems in such a way that in the progressive zone the lines of constant power are mainly horizontal. Reference is made to DE-AS 26 10 203 with regard to the advantages of such a course for the lines of constant power.
In addition, the progressive spectacle lens in accordance with the present invention has the surprising advantage that distortion is low.
The idea in accordance with the present invention of transposing the irritating surface astigmatism of a progressive spectacle lens into that area which is dispensed with for half-eye spectacles by means of mathematical methods permits extremely great freedom in design.
The main meridians of the surfaces which contribute to the power increase can be formed in a known fashion as umbilical lines or show astigmatism. It is, however, possible to compensate for this astigmatism as well as for any astigmatism which may be present in the progressive corridor by means of a counter-surface which also has an aspheric design.
The main meridians may be winding, i.e. the main meridian curves are not in one plane; the main meridian curves may, however, also be plane and the lens fitted into the spectacle frame in a rotated position.
Of course, it is also possible to calculate the spectacle lens according to the present invention which is provided with a winding main meridian, or which is fitted into a spectacle frame in a rotated position, in such a way that it is horizontally symmetrical in the fitted position.
It is, however, of particular advantage that the large reading portion which is practically free of astigmatism and the wide progression corridor in which clear vision is possible permit the lens to be fitted into the spectacle frame without rotation and without this resulting in any physiological impairments.
In this way a horizontally symmetrical lens is obtained without any great effort in the calculation process which could only be obtained with great effort in the calculation process when fitted in a rotated position or with a winding main meridian and which would also require the surfacing of so-called "left" and "right" lenses.
Surprisingly, it was found that such a progressive spectacle lens for half-eye spectacles will not only be accepted very well by emmetropic presbyopes, i.e. by persons who require no correction for distance vision, but also by presbyopes with slight vision impairments.
The surfaces described in the above-named older patent application Ser. No. P 34 30 334.0 are characterized in the progressive corridor by the fact that the course of the curvature radii of the so-called orthogonal sections deviates completely from that stated, for example, in the U.S. Pat. No. 2 878 721 or in DE-AS 20 44 639. In accordance with this older suggestion, the curvature radius of the orthogonal sections increases in the vicinity of the main meridian on the front surface and decreases on the back surface, while in the case of the spectacle lens described in patent application Ser. No. P 34 30 334.0 it decreases and increases respectively.
In any case, it is particularly advantageous if at the connection to that area in which the power increases progressively, the spectacle lens has an area of constant or practically constant power which should have a practically spherical design and therefore free of astigmatism. In addition, it is also possible to provide a reading portion with toric power for spectacle wearers with near astigmatism.